Behind blunder, stop working on trousers (8)
I believe the answer is:
derriere
'behind' is the definition.
(synonyms)
'blunder stop working on trousers' is the wordplay.
'blunder' becomes 'err' (to err is to make a mistake or blunder).
'stop working' becomes 'die' (I've seen this in other clues).
'on' becomes 're' ('on' can be mean 'with reference to').
'trousers' is an insertion indicator (trouser can mean to steal or take).
'die'+'re'='diere'
'err' inserted inside 'diere' is 'DERRIERE'.
(Other definitions for derriere that I've seen before include "(French?) behind" , "Seat" , "A French backside or buttocks" , "One's backside in France" , "Fourteen down's [FRENCHMAN] behind" .)